The invention relates to devices for optical signal transmissions between a transmitter unit and a mobile receiving unit, which are optically coupled to each other via an optical transfer medium.
Optical systems are frequently employed to transmit data and signals. Such systems are fundamentally composed of a transmitter unit and a receiver unit, which are interconnected via an optical transfer medium. When the optical transfer medium is free space or air, an arrangement similar to a light barrier is achieved.
However, optical fibers, such as glass or synthetic fibers, are more frequently used to guide the light. In both case, the length of the optical path between the transmitter unit and the receiving unit is generally constant. As a result, the amplitude of the signal received in the receiver unit is not subject to significant variations with respect to time. This facilitates a uniform transmission quality.
In the case of transmission paths that have a variable optical path length between the transmitter unit and the receiving unit, the signal level at the receiver may vary due to attenuation along the optical path, which in turn may degrade transmission quality. In advanced digital transmission systems, in particular, this may result in an undesirable increase of the bit error rate.
The transit time of optical signals through the optical transfer medium may vary depending upon the distance between the transmitter and the receiver, varying from a range of almost zero when the transmitter is located in the immediate proximity of the receiver, up to a maximum value occurring when the transmitter unit is located at the farthest point along the optical medium from the receiver.
When the transmitter moves along the length of the medium, starting from the receiver up to the end of the optical medium, the transit time of the optical signal will increase as the transmitter moves away from the receiver. However, in rotary systems, the transmitter initially begins transmission at a starting point that is located at the receiver and continues around the circumference of the optical medium, thereby increasing the optical path length. Once the transmitter reaches the end of the optical transfer medium, the longest optical path length, the transmitter will then immediately transition from the end of the optical medium to the initial starting point to begin the process again. At the end of the optical transfer medium, there is a relatively long transit time for the optical signal to reach the receiver due to the distance traveled by the optical signal. However, upon transition to the starting point, the transit time is immediately reduced to almost zero.
This abrupt difference in transit time, which may occur during the transition, may give rise to a discontinuity in phase, restricting the bandwidth that can be transmitted, and may result in transmission errors.
Particularly, when optical signals are transmitted via an optical transfer medium shaped in the form of a closed curve, an overlapping at the beginning and the end of the optical medium is unavoidable unless a gap in transmission can be accepted in this position. That is, two signals are superimposed in the receiver, at the beginning and simultaneously at the end of the medium. The first signal is transmitted to the receiver without traveling along the optical path, and therefore reaches the receiver almost immediately. The second signal passes over the entire optical path length, and thus arrives at the receiver with a substantial delay. Both signals are now superimposed and produce an incorrect cumulative signal. As a result, the transmitted signal is degraded. Specifically, with high frequencies where the signal transit time corresponds to one half of the period, the signal is extinguished, such that a sensible data transmission is no longer possible.
What is desired then, is an optical transmission system that provides for transmission of optical signals from a transmitter to a receiver along a rotatable optical coupling where the optical signal quality is not degraded.
It is further desired to provide an optical transmission system having a signal transmission quality independent of the relative movements between the transmitter and the receiver.
It is also desired to provide an optical transmission system in which no signal overlapping occurs at the receiver to interfere with data transmission.
It is further desired to provide an optical transmission system that is small in size, is cost effective, and may be utilized for wide-band signal transmission.